This invention relates to methods and apparatus for determining the composition of subterranean formations and of materials in boreholes traversing such formations; and in particular to such methods and apparatus using spectral analysis of nuclear radiation emanating from the formations and materials.
In nuclear well logging, fast (14 MeV) neutrons are used to bombard the formation adjacent a borehole. Inelastic and capture gamma radiation resulting from interactions between the neutrons and the formation constituents are detected and recorded. Spectral analysis is then used to determine from the recordings the constituents of the formation and borehole. Thus, as described in U.S. Pat. No. 3,521,064 to Moran et al, the detected gamma ray spectrum for a formation of interest is compared with a composite spectrum made up of standard spectra of the constituents postulated to be present in the formation, each standard spectrum being weighted by a corresponding coefficient. The weighing coefficients of the standard spectra are calculated to give a best fit of the composite spectrum to the detected spectrum. These best fit coefficients represent the relative contributions of the corresponding constituents to the total gamma ray spectrum, and they are functions of the relative abundances of those constituents in the formation. Appropriate selection of the standards to be included in the analaysis enables the relative proportions of the constituents of interset, such as carbon, oxygen, chlorine and hydrogen, to be obtained and the desired information regarding oil content to be derived.
In order to penetrate the subterranean formation the fast neutrons must pass through the fluid contents of the borehole before entering the formation. The resulting borehole contributions to the inelastic and capture gamma ray spectra significantly complicate the analysis of the formation composition. One way of accounting for these contributions is to claibrate the logging tool in a reference borehole having known borehole contents and formation compositions. However, this requires a large number of calibration measurements. Also laboratory conditions do not necessarily reflect the real composition of the contents of the borehole, so inaccuracies can result in the constituent proportions obtained from the spectra matching process.
Taking more accurate account of the composition of an individual borehole's contents would enable more accurate information to be obtained concerning the constituents of the earth formations surrounding a borehole. Although the composition of the contents of the borehole may be determined with other logging tools, the use of the logs from such tools to correct the spectral analysis results requires accurate recording of the measurements and of the corresponding positions along the borehole. Separate borehole passes may be required for each measurement, contributing further to errors which arise from merging the data to assure depth correspondence. Each additional log requires additional expense and delay and contributes further errors.
U.S. Pat. No. 4,507,554 to Hertzog and Nelligan, assigned to the assignee of this application, discloses a method of determining the composition of the borehole material in which an inelastic spectrum is recorded during the neutron burst and two capture spectra are obtained in respective timer periods following the burst; one shortly after the burst and a second a much longer time after. The recorded spectra are analysed as described above using sets of standard spectra specific to each time period. It is assumed that the earlier of the two capture spectra contains information about both the borehole and the formation, whereas the later capture spectrum contains information only, or at least primarily, about the formation. Accordingly, the difference between the constituent analyses derived from the capture spectra is taken to indicate the composition of the borehole. This technique has the disadvantage that the time period between successive neutron bursts may be relatively long, to allow the radiation emanating from the borehole constituents to subside sufficiently before the second capture spectrum is recorded. Consequently the logging speed must be relatively low, or alternatively poor depth resolution of the logs must be accepted. In addition, the assumption of little or no borehole contribution to the second capture spectrum is only an approximation, and thus does not necessarily reflect the real environment in which the spectral measurements are made.
It is an object of this invention to provide an improved method and apparatus for determining borehole and formation constituents.